Each input line is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. Any filename of the form var=value is treated as an assignment, not a filename, and is executed at the time it would have been opened if it were a filename. Variables assigned in this manner are not available inside a BEGIN rule, and are assigned after previously specified files have been read.

An input line is normally made up of fields separated by white spaces. (This default can be changed by using the FS built-in variable or the -Fc option.) The default is to ignore leading blanks and to separate fields by blanks and/or tab characters. However, if FS is assigned a value that does not include any of the white spaces, then leading blanks are not ignored. The fields are denoted $1, $2, ...; $0 refers to the entire line.

A pattern-action statement has the form:

pattern { action }

Either pattern or action may be omitted. If there is no action, the matching line is printed. If there is no pattern, the action is performed on every input line. Pattern-action statements are separated by newlines or semicolons.

Patterns are arbitrary Boolean combinations ( !, ||, &&, and parentheses) of relational expressions and regular expressions. A relational expression is one of the following:

expression relop expression expression matchop regular_expression

where a relop is any of the six relational operators in C, and a matchop is either ~ (contains) or !~ (does not contain). An expression is an arithmetic expression, a relational expression, the special expression

var in array

or a Boolean combination of these.

Regular expressions are as in egrep(1). In patterns they must be surrounded by slashes. Isolated regular expressions in a pattern apply to the entire line. Regular expressions may also occur in relational expressions. A pattern may consist of two patterns separated by a comma; in this case, the action is performed for all lines between the occurrence of the first pattern to the occurrence of the second pattern.

The special patterns BEGIN and END may be used to capture control before the first input line has been read and after the last input line has been read respectively. These keywords do not combine with any other patterns.

Built-in variables include:

FILENAME

name of the current input file

FS

input field separator regular expression (default blank and tab)

NF

number of fields in the current record

NR

ordinal number of the current record

OFMT

output format for numbers (default %.6g)

OFS

output field separator (default blank)

ORS

output record separator (default new-line)

RS

input record separator (default new-line)

An action is a sequence of statements. A statement may be one of the following:

if ( expression ) statement [ else statement ] while ( expression ) statement do statement while ( expression ) for ( expression ; expression ; expression ) statement for ( var in array ) statement break continue { [ statement ] ... } expression # commonly variable = expression print [ expression-list ] [ >expression ] printf format [ ,expression-list ] [ >expression ] next # skip remaining patterns on this input line exit [expr] # skip the rest of the input; exit status is expr

Statements are terminated by semicolons, newlines, or right braces. An empty expression-list stands for the whole input line. Expressions take on string or numeric values as appropriate, and are built using the operators +, -, *, /, %, ^ and concatenation (indicated by a blank). The operators ++, --, +=, -=, *=, /=, %=, ^=, >, >=, <, <=, ==, !=, and ?: are also available in expressions. Variables may be scalars, array elements (denoted x[i]), or fields. Variables are initialized to the null string or zero. Array subscripts may be any string, not necessarily numeric; this allows for a form of associative memory. String constants are quoted (""), with the usual C escapes recognized within.

The print statement prints its arguments on the standard output, or on a file if >expression is present, or on a pipe if '|cmd' is present. The output resulted from the print statement is terminated by the output record separator with each argument separated by the current output field separator. The printf statement formats its expression list according to the format (see printf(3C)).

The arithmetic functions are as follows:

cos(x)

Return cosine of x, where x is in radians. (In /usr/xpg4/bin/awk only. See nawk(1).)

sin(x)

Return sine of x, where x is in radians. (In /usr/xpg4/bin/awk only. See nawk(1).)

exp(x)

Return the exponential function of x.

log(x)

Return the natural logarithm of x.

sqrt(x)

Return the square root of x.

int(x)

Truncate its argument to an integer. It will be truncated toward 0 when x > 0.

The string functions are as follows:

index(s, t)

Return the position in string s where string t first occurs, or 0 if it does not occur at all.

int(s)

truncates s to an integer value. If s is not specified, $0 is used.

length(s)

Return the length of its argument taken as a string, or of the whole line if there is no argument.

split(s, a, fs)

Split the string s into array elements a[1], a[2], ... a[n], and returns n. The separation is done with the regular expression fs or with the field separator FS if fs is not given.

sprintf(fmt, expr, expr,...)

Format the expressions according to the printf(3C) format given by fmt and returns the resulting string.

substr(s, m, n)

returns the n-character substring of s that begins at position m.

The input/output function is as follows:

getline

Set $0 to the next input record from the current input file. getline returns 1 for successful input, 0 for end of file, and -1 for an error.

See largefile(5) for the description of the behavior of awk when encountering files greater than or equal to 2 Gbyte ( 2³¹ bytes).

length > 72

{ print $2, $1 }

BEGIN { FS = ",[ \t]*|[ \t]+" } { print $2, $1 }

{ s += $1 } END { print "sum is", s, " average is", s/NR }

{ for (i = NF; i > 0; --i) print $i }

/start/, /stop/

$1 != prev { print; prev = $1 }

/Page/ { $2 = n++; } { print }

Assuming this program is in a file named prog, the following command line prints the file input numbering its pages starting at 5:

awk f prog n=5 input